The present invention relates to heat exchangers. In particular, it relates to a multi tube heat exchanger for exchanging heat between two fluids.
Heat exchangers are used in many industries including food industry. A variety of heat exchangers are used depending upon specific application and so plate heat exchangers, tubular heat exchangers, shell and tube heat exchangers and scraped surface heat exchangers, etc. are used widely in the food industry. Prior art heat exchangers are efficient and cost effective when the fluids passing through them are Newtonian flow and have low viscosities. The capital and operating cost effectiveness depends in large part on the ability to use small diameter tubes to improve heat transfer to the tube side fluid.
However, a number of food products are thick Newtonian fluids, showing linear relationship between shear stress and shear rates and having very high apparent viscosities. Also, a large number of food slurries are non-Newtonian liquids with a very high apparent viscosity. Because of high viscosities, the operating pressure drop through conventional heat exchangers with small tubes rises to uneconomic levels, making high flow very costly due to pumping power and capital costs. The higher pressure drop requires one or more positive displacement pumps, which increase both the capital and operating costs.
One prior art method to reduce high operating pressures is to reduce product flow rates through small tube heat exchangers by arranging product flow in many parallel streams. Lower flow rates on the other hand result in lower heat transfer rates that requires an uneconomic increase in large heat transfer area. Increasing heat transfer area results into more capital investment, more space requirement and significant product loss in cleaning, a rather frequent requirement for food processing. Further, reducing flow rates for non-Newtonian fluids causes higher apparent viscosities at operating pressures, temperatures and flows. Therefore, for non-Newtonian fluids, lowering flow rates does not significantly reduce operating pressure drops.
The xe2x80x9ccoring effectxe2x80x9d is the tendency in a double tube heat exchanger (with the product carried in the inner tube and the cooling/heating fluid in the annular space between the inner and outer tube) for a central portion of the product stream in the inner tube to move faster than the rest of the material at the tube boundary where heat transfer takes place. The central portion or the xe2x80x9ccorexe2x80x9d of the product does not experience any significant mixing and so heat transfer is decreased. The coring effect is more pronounced as the tube diameter of the inner tube is increased and the viscosity of the product increased. Turbulators are inserted to disturb the central portion of thick product flow in the inner tube by creating turbulence. These turbulators have different shapes from an augur or spiral like shapes and run through the length of the inner tube. Another type of turbulator is simple smooth surface insert in the second tube, i.e., a tube turbulator, occupies a small cross section area at the center of the second tube. Placement of tube turbulators thus creates an annular space through which the product flows, but the prior art devices provide for a very wide annular space. For example, a tubular heat exchanger with 4xe2x80x3 outer tube and 3xe2x80x3 inner tube may have a tube core of xc2xexe2x80x3 diameter. Thus it will create an annular space of 1.08xe2x80x3 which is very wide. Application of these cores are limited to double tube heat exchangers with inner tube of larger diameter and are not placed inside an inner tube with say 1xe2x80x3 diameter as xe2x80x9ccoringxe2x80x9d or xe2x80x9clayeringxe2x80x9d effect is very insignificant as the inner tube diameter decreases.
Multi-tube heat exchangers provide more heat transfer surface as it employs a bundle of small diameter tubes through which product flows and an outer tube enclosing the bundle of small tubes. It makes the heat exchanger more compact than the double tube heat exchanger and thus require a smaller foot print. The outer shell diameter could vary from 3xe2x80x3 to 8xe2x80x3 while the inner small tubes are predominantly of xc2xdxe2x80x3 to xc2xexe2x80x3 diameter and up to 1xe2x80x3 in some cases. These heat exchangers are widely used for heat exchange applications in food industry but they do not work well for thicker food products especially those having very high viscosity at lower shear rate because very high pressure drops are developed and typical slow product velocity results into lower heat transfer.
Scraped surface heat exchangers handle this type of fluids very efficiently but again they are expensive and also require constant maintenance. Some other kinds of tubular heat exchangers with corrugations are offered but they are difficult to clean as they do not drain well and also more expensive because of special. design and fabrication.
One application of tubular heat exchangers in food industry is for energy regeneration where heat exchange takes place between a hot and cold product streams which is termed as xe2x80x98product-to-productxe2x80x99 regeneration or direct regeneration. Direct regeneration for thin products is usually carried out employing a double tube or a triple tube heat exchanger. Direct regeneration becomes increasing difficult as the product viscosity increased as prevailing laminar flow situation not only drastically reduces overall heat transfer coefficient but also difficult to clean in place as uneven velocities of cleaning solutions. For these types of application, an indirect regeneration or xe2x80x98water-to-productxe2x80x99 regeneration is employed where water in close loop recovers heat energy from a hot product in a cooling regenerator and gives back this heat energy to cold product stream in a heating regenerator. Double tube heat exchangers or multi-tube heat exchangers are used for indirect regeneration. A lower heat transfer rate and high pressure drop limitations for a thick non-Newtonian fluid results in a large heat exchanger surface requirement. A triple tube heat exchanger is not preferred for this types of heat application even though it employs product annular space.
Triple tubes and double tubes have a large exposed surface to heat transfer surface ratio which means that heat loss and refrigeration loss to surroundings is high if the tubes are not properly insulated. This ultimately results in lower thermal effectiveness in comparison to plate heat exchanger and multi-tube heat exchangers. For example a 2xc2xdxe2x80x3xc3x971xc2xdxe2x80x3 double tube has exposed surface to heat transfer area ratio of 1.66 while a multi tube heat exchanger with 2xc2xdxe2x80x3 outer tube and xc2xdxe2x80x3 inner tubes will have this ratio as 0.71. The insulation costs are therefore higher in tubular heat exchanger as compared to multi tube heat exchangers.
In triple tube heat exchanger employed as a regenerator, there are two annular cross sections and one circular cross section through which the hot and cold streams of product flow. In U.S. Pat. No. 3,386,497, a hollow core tube has been inserted in inner round tube of a triple tube regenerator for thick food products like tomato paste to reduce xe2x80x9clayeringxe2x80x9d or xe2x80x9ccoringxe2x80x9d of the product in round cross section of the inner tube. The use of tube turbulators or cores in double tube heat exchanger and triple tube heat exchanger, as described in U.S. Pat. No. 3,386,497, changes the product flow from xe2x80x9cflow through round cross sectionxe2x80x9d into xe2x80x9cflow through an annular cross sectionxe2x80x9d and thus reaps the benefits of superior heat transfer characteristics of an annular space. However, this arrangement does not address the issue of making tubular heat exchanger more compact. Bulkiness is one of the inherent limitations of tubular heat exchanger and this limitation further gets amplified when thick food products are handled by tubular heat exchangers. This patent shows an example of the prior art thinking to use a tube turbulator, although it is apparent that tube turbulators have not been advanced as a technical improvement since the 1968 date of this patent. The more recent and consistent approach to this problem of improving economically effective flow rates and heat transfer surface areas are shown in U.S. Pat. Nos. 3,921,711 and 4,593,754 where extremely irregular surfaced turbulators are used to increase turbulence. The problem of cleaning the triple tube exchanger is illustrated in U.S. Pat. No. 4,679,622. For heat exchangers with small hydraulic cross sections, an economically adequate velocity above about 3 ft/sec or higher is needed for cooling thick food slurries and would result in extremely high pressure drops through these heat exchangers, raising pumping costs to impossibly expensive levels. The slurry side tubes in non-insert exchangers used for cooling thick food slurries have an inner diameter of 12 mm and larger due to the high capital and utilities costs for smaller diameter tubes, where the necessarily slow velocities result in flow in laminar region.
An object of the present invention is to provide a heat exchanger which works well for non-Newtonian especially shear thinning food slurries like tomato ketchup, concentrated fruit juices, sauces, fruit purees etc. achieving a higher beat transfer rates with comparatively lower pressure drops than the known tubular heat exchangers. This will reduce the capital investment.
Another object of the present invention is to provide heat exchanger that is compact in size, requiring less floor space.
Another object of the present invention is to provide heat exchanger, which holds lower volume of product and thus minimizes product loss.
Another object of the present invention is to provide a heat exchanger that lowers heat and refrigeration loss and also costs less to insulate.
Another object of the present invention is to provide a heat exchanger, which is easy to clean and maintain.
Another object of the present invention is to provide a heat exchanger, which can be easily customized to permit optimization of flow velocity and heat exchanger area for a variety of applications.
The foregoing objects are accomplished by the present invention, which is a multi tube heat exchanger but unlike a conventional multi tube heat exchanger, where the product flows through the smaller tube bundles, here the product flows in the multiple annular spaces while the medium flows in the shell side. The heat exchanger comprises of three main sections: middle section, inlet chamber at one end of the middle section and an outlet chamber at the opposite end of the middle section. The middle section is like shell and tube heat exchanger having two lengths of a large tube connected by an expansion joint near one end. Middle section has one tube sheet welded at each end and a numbers of small tubes whose both ends are welded or expanded into these tube sheets. The tube ends are flush with the tube sheet. The outermost tube of the middle section has connections for inlet and outlet for the medium.
The inlet and outlet flow chambers are identical in construction except that the inlet and outlet connections are in opposite ends to each other or in any suitable place for appropriate product and media connection. The chambers have a tube section with one end having a matching flange that fits with the tube sheet of the middle section at one end. The other end of the chamber has a tube sheet having number of bores equal to those of the tube sheets on the middle section and this tube sheet connects to another tube sheet with matching number of bores. The bores in the tube sheet at the end of flow chamber are larger than the core diameter, widening further towards the matching tube sheet. This tube sheet has holes, which are smaller than the tube sheet on the chamber but large enough to easily pass the cores through them. Both these tube sheets are held together with a quick release sanitary clamp. A number of smaller core elements pass through these tube sheets at one end of the module, through the small tubes in the middle section and through similar chamber on the opposite side of the middle section of the module. Each core element is either a solid round bar or a smooth tube or pipe. Each tube in the middle section thus has one smaller core element in the center. These cores are smaller in diameter than the tubes in the middle section and thus form a number of annular spaces equal to the number of small tubes passing through the middle section. A gasket fits into each annular tapering space between core passing through the hole in the tube sheet of flow chambers at both ends of the module. These gaskets are pressed against the core surface and tube sheet by the last tube sheet when both tube sheets at the end of the module are connected together by a quick release clamp. A leak proof joint is thus formed which prevents leakage of fluid from the chamber to outside.
Product enters the heat exchanger through the inlet port of the inlet chamber, pass through the annular spaces through the middle section and come out of the opposite end through outlet chamber. Medium on the other hand enters middle section through the inlet port which is at the outlet chamber end, pass through the shell side and come out of the other end of middle section, thus forming a counter current flow between product.
The rate of heat transfer between a thick liquid food and heating or cooling medium in a thin walled heat exchanger is a special case in that the over-all-heat transfer coefficient is mainly governed by the heat transfer coefficient on the product side. This is so because the product flow is mostly is laminar as a result of the very high viscosity of the product, while media side is always designed to have turbulent flow. Heat transfer rate in this case can only be increased if the heat transfer coefficient on the product side is increased. Higher shear rates in annular spaces as encountered in the present invention increases heat transfer coefficients. Higher shear rates also results in lower pressure drops as explained in following paragraphs.
Because of the geometry of the annular space, the distance-from the maximum velocity region which lies somewhere near the center, to the wall where the velocity is zeroxe2x80x94is less in comparison to the circular cross section tube of the same cross sectional area. And so, shear rates in an annular space are higher than in circular space of the same cross sectional area. Depending upon the diameter ratios of the tubes and flow behavior index of the fluid, the shear ratios can be as high as 2 to 2.5 times that in circular cross sections. Now, non-Newtonian fluids especially shear thinning foods (most foods fall in this category) show a lower apparent viscosity at higher shear rates than at lower shear rates. It follows from this that due to higher shear rates in the new design results into a lower pressure drop for a given flow rate.
The superior heat exchange efficiency of annular space is known to prior art and so is widely used in food industry in the form of triple tubes for Newtonian fluids with lower viscosity values for products like milk and juices. Its use for very thick Newtonian and non-Newtonian fluids is limited at present because of very high pressure drops encountered in these heat exchanger in the present form as the known design does not permit an optimum combination of shear rates and pressure drops. Another reason is that application of triple tubes for this type of application becomes more expensive. One reason for the high cost of triple tube is that it requires a third larger tube around each product annulus. In the present invention the cost is reduced by elimination of separate outer tube on each annulus by a single large shell surrounding all annuli. This arrangement provides for a large heat exchange areas and makes this beat exchanger very compact.
Since, for a given product flow rate higher heat transfer rates are achieved, a smaller heat transfer area is required which makes the heat exchanger further compact. Since for any given flow rate and heat transfer area pressure drops are lower than the known tubular heat exchanger designs, product flow rate could be increased which further increases heat transfer rates. Thus higher heat transfer rate and lower pressure drops for a specific heat transfer application, result in a heat exchanger that is more compact and requires less floor space. In short, the new design offers the superior heat exchange capability and low pressure advantage of the product annular space as in a triple tube and the compactness and of a multi tube heat exchanger.
Since the new design requires less heat transfer area, number of tube required is reduced and so the product hold up in these tubes is reduced.
Tubular heat exchangers have comparatively large areas exposed to surrounding which results in a higher heat transfer between heat exchanger and surrounding. This results into substantial heat losses or refrigeration losses, which not only means higher energy requirement but also means a lower higher thermal effectiveness and larger approach temperature. The present invention by way of its compactness lowers down the exposed surface resulting into lower cost of insulation and a better thermal efficiency.
The present design is simple in comparison to the scraped surface heat exchangers as there are no moving mechanical parts and hence it is easy to maintain. Further product flows without any obstructions and so it is easier to clean also. Also, the inner surface of the tube outer tube in the annular space is flush with the inner surface of the inlet and outlet chambers, which facilitates easy draining of the tubes.
All these improvements in the performance of the heat exchanger make it very suitable for product like tomato paste, heavy milk cream, concentrated fruit juices and sugar syrups etc.